This application is based on application No. 2008-159067 filed on Jun. 18, 2008 in Japan, the content of which is hereby incorporated by reference.
(1) Field of the Invention
The present invention relates to a fixing device that fixes an unfixed image formed on the recording sheet such as a sheet of recording paper or an OHP sheet, and an image formation apparatus including the fixing device. The present invention particularly relates to technology that prevents creases of the recording sheet when the fixing device fixes the unfixed image with use of a fixing belt.
(2) Description of the Related Art
An image formation apparatus such as a photocopying machine is equipped with a fixing device that fixes an unfixed image formed on a recording sheet such as a sheet of recording paper or an OHP sheet. An example of such a fixing device is a belt-nip type fixing device. The belt-nip type fixing device includes a fixing roller that rotates with the surface thereof heated to a predetermined temperature, and an endless fixing belt. The belt-nip type fixing device has a structure in which the fixing roller and the fixing belt are mutually brought into pressure-contact with each other so as to form therebetween a fixing nip portion through which the recording sheet passes. The fixing belt that is brought into pressure-contact with the fixing roller rotates according to the rotation of the fixing roller. While the recording sheet on which the unfixed image has been formed passes through the fixing nip portion between the fixing roller and the fixing belt, the unfixed image formed on the recording sheet is heated and applied with pressure so as to be fixed on the recording sheet.
Japanese Laid-open patent application publication No. 2003-5553 discloses a structure in which a pressing member provided inside a closed rotation path of the fixing belt presses the rear surface of the fixing belt such that the surface of the fixing belt is brought into pressure-contact with the surface of the fixing roller. The pressing member is composed of a part having rigidity so as to press the fixing belt entirely in the width direction thereof with a predetermined pressure. The part having rigidity presses the fixing belt towards an axis (rotation center) of the fixing roller. This applies uniform pressure to the surface of the fixing roller along the rotating direction.
However, in the belt-nip type fixing device, speed at which the recording sheet is conveyed through the fixing nip portion between the fixing roller and the fixing belt possibly becomes uneven in the width direction of the fixing belt depending on the states of the fixing roller, the fixing belt and the pressing member, and the moisture state of the recording sheet, for example. This possibly causes creases of the recording sheet entering the fixing nip portion.
In the structure in which the pressing member presses the fixing belt, it is not easy to vary the speed of conveying the fixing belt in the width direction thereof. As a result, a problem arises that the creases of the recording sheet that passes through the fixing nip portion is not easily prevented.
Unlike the belt-nip type fixing device, in the fixing device in which the fixing roller is brought into pressure-contact with a pressure application roller, it is known that the creases of the recording sheet that passes through the fixing nip portion can be prevented by making a conveyance speed at which the recording sheet passes through the fixing nip portion faster in each edge portion of the fixing roller in the width direction thereof than in a central portion of the fixing roller in the width direction thereof. However, as disclosed in Japanese Laid-open patent application publication No. 2003-5553, in the belt-nip type fixing device in which the fixing belt pressed by the part having rigidity is brought into pressure-contact with the fixing roller to form the fixing nip, it is not easy to make the speed of conveying the recording sheet faster in each end portion of the fixing belt in the width direction thereof than in the central portion of the fixing belt in the width direction thereof. That is, in order to make the speed of conveying the recording sheet in the fixing nip portion faster in each end portion of the fixing belt in the width direction thereof than in the central portion of the fixing belt in the width direction thereof, it is necessary to deform the part having rigidity into a complicated shape, for example. Therefore, it takes a long time to manufacture such a part having rigidity, which is not economically efficient.